Visbreaking is a thermal cracking process for converting heavy asphalt-like distillates into lighter, more commercial products, such as fuel oil. Visbreaking is accomplished in a furnace operating at pressures of from 400 to 1,600 psig, and temperatures of 700.degree. to 900.degree. F.
Effluent from the furnace is passed through a letdown valve, where the pressure and temperature is drastically reduced. The severe reduction of the pressure and temperature makes extreme requirements upon the design and operation of the letdown valve. If the letdown valve is not designed properly, the flow of the furnace effluent will back-up into the furnace or the valve will fail.
The letdown valve of this invention depressurizes and cools the furnace effluent, and maintains a smooth flow of the liquid such that noise and turbulence are reduced as compared to other valve types.
As the effluent passes through the throat of the valve, the liquid is throttled, and a portion is caused to flash to vapor, the vapor having a greater volume than the liquid, is then the controlling factor in the proper hydrodynamic design of the valve. In order to obtain continuous back-up-free flow, this invention features vapor leaving the outlet of the valve at sub-sonic speed. The invention has designed the outlet diameter of sufficient width to obtain subsonic flow under all process conditions.
The invention also features a unique quenching design for quenching the high temperature liquid.